I am no expert on this, lol although i have built a single Xeon Dual core server, and a Dual Quad core Server using the same dual socket supermicro boards. But i would imagine as long as it is the same processor you use in the 2nd socket you should be fine.
all 5120 processors should be the same specification, as each different specification processor is denoted by a different number, as shown below.
Quad-Core Intel® Xeon® processor 5400 series:
X5460 SLANP 3.16 GHz 1333 MHz 12 MB C0
X5472 SLASA 3.00 GHz 1600 MHz 12 MB C0
X5450 SLASB 3.00 GHz 1333 MHz 12 MB C0
E5472 SLANR 3.00 GHz 1600 MHz 12 MB C0
E5450 SLANQ 3.00 GHz 1333 MHz 12 MB C0
E5440 SLANS 2.83 GHz 1333 MHz 12 MB C0
E5462 SLANT 2.80 GHz 1600 MHz 12 MB C0
E5430 SLANU 2.66 GHz 1333 MHz 12 MB C0
E5420 SLANV 2.50 GHz 1333 MHz 12 MB C0
E5410 SLANW 2.33 GHz 1333 MHz 12 MB C0
E5405 SLAP2 2.00 GHz 1333 MHz 12 MB C0
So as long as you get another 5120 Xeon it should be fine, if the board was designed for that processor, and has 2 sockets, i dont see any reason why you should have any issues.
But usually as far as i know, and could be wrong, the reason you need 2 processors which are the same type is because of the way they operate, as the 2 processors dont run 100% independant, they communicate with one another, which is why.. from reading around there is only 771 pins on a LGA socket Xeon compared to 775 on a Core 2 Duo or similar. So the other 4 pins arent required as whatever they are used for is shared between the 2 processors. Its the same with Dual channel RAM and Fully buffered, has to be pairs to work properly, or with performance increase.
But as i said, im not 100% so somebody else might be able to offer more definite advice.
Hope i have been of some help though.
